Remove duplicate rows

Hello,

I need to remove duplicates rows from a subquery but order these
results by a column what is not selected. There are logically two
solutions but no works.

SELECT DISTINCT sub.foo FROM (SELECT ...) AS sub ORDER BY sub.bar
ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

SELECT sub.foo FROM (SELECT ...) AS sub GROUP BY sub.foo ORDER BY sub.bar
ERROR: column "sub.bar" must appear in the GROUP BY clause or be used
in an aggregate function

Does anybody know how to remove duplicate rows from a subquery and order
these results by a column what is not selected but exists in a subquery?

Thanks for any advice,

J.N.